1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, an MFP, a facsimile machine, or a multifunction device thereof, and a method for detecting a pressure contact or separated state of a transfer unit in the image forming apparatus. For example, the present invention can be utilized for detecting a pressure contact or separated state of a secondary transfer roller with respect to an intermediate transfer belt.
2. Description of the Prior Art
Conventionally, a copying machine of an electrophotographic method, a printer, a facsimile machine, or an image forming apparatus such as a multifunction device called an MFP (Multi Function Peripherals) forms images by developing an electrostatic latent image formed on a photosensitive drum so as to form a toner image, which is transferred to an intermediate transfer belt as a primary transfer and further transferred to a paper sheet as a secondary transfer, which is fixed. In order to perform the secondary transfer of the toner image from the intermediate transfer belt to the paper sheet, there is provided a secondary transfer roller that becomes a pressure contact state with respect to the intermediate transfer belt that is an image carrier.
In this image forming apparatus, the secondary transfer roller can move with respect to the intermediate transfer belt between the pressure contact state and the separated state. Although the secondary transfer roller is in the pressure contact state in a normal image forming (printing) state, it is usually in the separated state while the image forming process is not performed.
In another conventional structure, a test toner patch (an image stabilizer pattern) is formed on the intermediate transfer belt or the photosensitive drum, and a state of the toner patch is detected by an IDC sensor that is a density detector so that conditions for forming an image are adjusted. In this case too, the secondary transfer roller is usually set to be in the separated state so that the secondary transfer roller or the like does not become dirty with the toner.
Furthermore, a press and separation driving device is provided for moving the secondary transfer roller, and an optical sensor such as a photointerrupter is used for detecting whether or not the secondary transfer roller is switched securely to the separated state or the pressure contact state by the press and separation driving device.
However, if the photointerrupter is used for detecting the pressure contact state and the separated state, the number of components increases, which causes increase of cost. On the other hand, Japanese unexamined patent publication No. 2004-264455 discloses a device that does not include a special-purpose photointerrupter, but a photointerrupter for detecting a paper jam is also used for the above-mentioned purpose.
However, a single photointerrupter is shared for detecting timings of paper arrival and pass and for detecting the pressed or separated state of the secondary transfer roller in the above-mentioned conventional device. Therefore, it is necessary to use a special pre-transfer sensor flag for detecting arrival and pass timings of a paper sheet, so an operation of detecting a paper jam or the like may be subject to some constraints.